Actually a:b generates a vector. You could use it as index only because the (...) accepts a list also, e.g.
octave-3.0.3:10> a = [1,4,7]
a =
1 4 7
octave-3.0.3:11> b = [1,4,9,16,25,36,49]
b =
1 4 9 16 25 36 49
octave-3.0.3:12> b(a) # gets [b(1), b(4), b(7)]
ans =
1 16 49
Now, the a:b:c syntax is equivalent to [a, a+b, a+2*b, ...] until c, e.g.
octave-3.0.3:15> 4:7:50
ans =
4 11 18 25 32 39 46
which explains what you get in 0:pi/4:pi.
A lone : selects the whole axes (row/column), e.g.
octave-3.0.3:16> a = [1,2,3;4,5,6;7,8,9]
a =
1 2 3
4 5 6
7 8 9
octave-3.0.3:17> a(:,1) # means a(1:3, 1)
ans =
1
4
7
octave-3.0.3:18> a(1,:) # means a(1, 1:3)
ans =
1 2 3
See the official MATLAB doc on colon (:) for detail.